Abstract:

The Incomati River basin has been experiencing a decrease in river flows since the 1970’s. At the same time its catchment area has undergone severe land use changes and increased water abstractions. Further water abstractions are planned for the future and this will pose a threat to downstream users including the river’s estuary ecosystem. Studies that investigate the effect of different natural and human induced factors to the downstream flow regime are lacking in the basin. Thus, the objective of this study was to assess the effect of land use changes and increased agricultural water abstractions on the lower Incomati River flow regime. The outputs from the study could be useful as relevant information in the negotiation of reallocation of water among the countries sharing the basin as well as in recommending appropriate IWRM practices to minimize the foreseen negative impacts.
Spearman Rank Correlation Test was used to examine the historical trend in streamflows and agricultural water abstractions. The Geospatial Stream Flow Model (GeoSFM) was integrated with remote sensing and GIS to model the impact of land use changes on river flow regime. Spearman Rank Correlation Coefficients of Rsp=-0.57 and Rsp=1 confirmed changes on streamflows and in agricultural water abstraction at the border gauge station of Ressano Garcia (E-23) with significance levels of 99.9% and 99% respectively. Land use maps of 1990, 2000, 2004 and 2010 derived from classification of Landsat TM images were fed into the GeoSFM and the results showed occurrence of significant changes (p<0.05) in streamflows in the sub-basins 26 and 33. These changes were more in terms of peak flows, time to peak and the response to rainfall events. The 2010 land use scenario produced hydrographs which responded faster to rainfall events than the 1990 land use. The peak flows generated by the 2010 land use hydrograph were of higher magnitude and steep rise to peak than those of 1990. These results suggest that the reduction in streamflows at the border station of Ressano Garcia can be attributed to other factors other than land use changes. Thus, IWRM strategies focusing on land use optimisation are recommended for further investigation and implementation in the affected sub-basins.